The usual flow control is a resilient (rubber) annulus which deflects under pressure to decrease the central opening. One attempt at controlling noise was to place a domed or bullet nosed diffuser downstream of and in line with the flow control orifice to cooperate with the deflecting flow control in a throttling action. But as the flow control came close there was a tendency for the flow control to close on the diffuser momentarily and then pop open. This "flutter" caused a noise similar to water hammer. That approach was abandoned.
Another attempted solution was the provision of a metal disc diffuser downstream of the flow control and provided with 3 or 4 holes located around the center so water flowing through the central flow control orifice was diverted to the holes in the diffuser. This proved to be subject to flutter as the flow control flattened on the diffuser. The flow between the flow control and diffuser also reached very high velocities with consequent noise.
Another approach provides the flow control with three orifices spaced around the center (no central orifice) and the underside is provided with slots leading from each orifice to the center where flow is passed into the orifice in the valve body. This eliminates flutter and keeps flow velocity more reasonable but is still noisy.